Cervical disc prostheses have been in clinical use for total disc replacement in the cervical spine for more than 10 years. Motion preservation or restoration after cervical discectomy by the use of a disc prosthesis is believed to be advantageous compared to fusion with respect to certain biomechanical considerations. Fusion of a cervical spinal motion segments leads to significant increase of intradiscal pressure at the adjacent segments. Therefore, fusion might be a trigger for accelerated adjacent segment degeneration. Fusion after cervical discectomy also decreases or at least changes the range of motion of the cervical spine to a greater extent than arthroplasty. Typically, patients are found to be back to their daily activities quicker after arthroplasty than after fusion.
It is generally accepted that a cervical disc prosthesis should replicate the kinematics of a natural cervical disc as closely as possible. The disc prosthesis should be able to follow the natural motion of the respective motion segment after its implantation. If the biomechanical design of a disc prosthesis is inadequate, then the motion segment may not move naturally but is forced to follow the biomechanics of the prosthesis. Such unnatural motion may cause increased stress to the facet joints and painful facet degeneration, further it may cause increased stress forces at the contact area of the prosthesis with the adjacent vertebral bodies endplates thus leading to implant migration into the vertebral bodies or even implant displacement. Further, it must be presumed that biomechanically inadequate disc prostheses also change the kinematics at the adjacent motion segments and therefore bear a similar risk for adjacent segment degeneration than conventional fusion.
Accordingly, before designing a cervical disc prosthesis, the kinematics of a natural cervical motion segment must be fully understood. Several independent motion properties are found in a cervical motion segment. There is flexion-extension motion which is coupled to mild anterior-posterior translation. Translation is mainly found in the cranial segments and gradually decreases from C3/4 to C6/7 (the C 2/3-motion-segment is mostly excluded in the respective studies in the literature, most probably because the indications for total cervical arthroplasty generally include the segments from C3/4 to C6/7). This motion pattern is determined by a center of rotation (COR) which is found approximately at the level of the upper endplate of the C7-vertebra at C6/7 and which gradually moves caudally for the cranial motion segments being approximately 12 mm below the upper C4-endplate for the C3/4-segment. In addition, there is coupled motion for side-bending and rotation between C3 and C7: every side-bending at these cervical motion segments also leads to ipsilateral rotation, and rotation leads to ipsilateral side-bending. This motion pattern is defined by a center of rotation or “COR” which is entirely independent from the previously described COR for flexion-extension and is found superior to the lower endplate of the upper vertebra of the respective motion segment. This coupled side-bending/rotation is facilitated around a longitudinal anterior-posterior axis through the respective COR following an oblique direction approximately crossing the anterior edge of the lower endplate of the upper vertebra and finally crossing the posterior edge of the upper endplate of the same vertebra.
In order to allow natural motion a cervical disc prosthesis must closely replicate the above mentioned biomechanical properties and must definitely allow motion through two independent CORs.
Moreover, the prosthesis COR for flexion-extension must be variable in order to replicate the different kinematics for the upper and the lower cervical spine. Finally, inter-individual differences for the respective patient's COR should be taken in account, and also the fact that candidates for cervical disc surgery often present degenerative changes at the entire cervical spine also affecting kinematics of their motion segments.
In order to achieve motion through two independent CORs, a three piece prosthesis with two endplates and an inlay offers greater flexibility with respect to its biomechanical design than a two piece construct. Such a three piece prosthesis with two independent gliding pairs—upper prosthesis-endplate versus upper surface of inlay, and lower surface of inlay versus lower prosthesis endplate potentially bears the property of including two independent motion patterns in a single prosthesis. Nevertheless, hardly any of the presently available three piece prosthesis fulfill the above mentioned biomechanical requirements. With exception of the Bryan-prosthesis, which closely replicates natural motion, most other three piece prostheses lack adequate biomechanical properties. None of the three pieces prostheses types disclosed in patent US 2010/0137992 A1 demonstrates a COR for side-bending/rotation above the implant as described above, neither a sufficiently variable COR for flexion/extension as described above. The two piece prosthesis which is disclosed in patent US 2010/0137992 A1 shows different diameters for flexion/extension and side-bending/rotation, however the COR for side-bending is below the implant and therefore may not provide natural motion. The drawings of patent US 2005/0228497 A1 show a disc prosthesis with several embodiments: some of the figures demonstrate unphysiological posterior translation in flexion and anterior translation in extension; other figures disclose saddle-like articulating surfaces allowing near—physiological translation with flexion/extension, but according to these drawings any rotation would cause cranio-caudal distraction of the implant which is entirely unphysiologic.
A brief overview of spinal anatomy and terminology will be beneficial in explaining one or more aspects of the inventions described herein. FIG. 1A shows a functional spinal unit from a lateral or sagittal view having a bony superior or upper vertebral body 1 having a vertebral endplate 2 connected to a bony inferior or lower vertebral body 4 via an intervertebral disc 3 comprised of an outer ring of fibrous collagen material (the annulus) surrounding an inner amorphous mass of material, the nucleus pulposus. Also shown are the posterior elements including the spinous process 5, pedicle 6, facet joint 7 and transverse process 8. FIG. 1B shows a vertebral body 1, 4 from a transverse plane view or an axial cross-section along the cranial caudal axis. The front or anterior portion of the vertebral body 1, 4 is curved and the posterior portion is relatively flat. Further posterior lie the facet joints 7 and other posterior elements and various ligaments (not shown).